This invention relates to a method and system for the detection of impurities in a flow of molten metal.
In the art of refining steel there exists the well known problem of minimizing slag carryover from one vessel to another, for example from a tundish to a mould, or as in the example noted below from a basic oxygen furnace to a ladle. In the refining process, raw steel, with certain impurities, is heated in a basic oxygen furnace (BOF) as pure oxygen is introduced. The impurities in the molten metal oxidize and form slag, leaving the purified molten metal. One of the properties of the slag is that it is less dense than the molten steel and as such it floats to the surface of the container. Traditionally, for pouring the molten metal, the BOF is tilted upon an axis and the container is tapped below a point where the slag should lie. In this manner, the refined steel can be poured into the ladle without the slag. As the amount of steel in the BOF is reduced, there is a greater probability that turbulent flow will cause slag carryover into the ladle. This is undesirable because of the great expense that must be incurred to remove the slag from the ladle, or to condition the metal so that the remaining slag is less reactive in later processes.
To prevent slag carryover, a method of slag detection is required so that when the detected slag exceeds a predetermined amount the pouring process is halted. Typically the pouring process is halted through closing the tap and the restoration of the BOF to an upright position. The process of detecting slag however still poses problems as a result of the environmental conditions of a steel foundry. The conditions in a steel mill do not easily lend themselves to human intervention in the pouring process, unless it is intervention from a distance. Another problem is that as a result of the high temperatures, molten steel appears similar to molten slag to the unaided eye of an operator.
Many techniques have been applied in an attempt to solve these problems, but the solutions are either ineffective or costly to implement, and in some cases both. Despite the fact that slag has a different consistency than molten steel, the high temperatures that are present heat both compounds and render the visual identification of the differences difficult for a human operator.
One currently used system is described in U.S. Pat. No. 5,968,227, to Goldstein et al, entitled xe2x80x9cSystem and Method for Minimizing Slag Carryover During the Tapping of a BOF Converter in the Production of Steelxe2x80x9d. This patent teaches the use of an infrared detection system to determine if slag has entered the stream between the BOF and ladle. The range of IR wavelengths selected for use in this invention is only minimally blocked by the water and carbon dioxide gasses that surround the steel making process. This allows the IR camera to be located far enough from the source of heat that background heat does not interfere with the imaging. When the camera images the stream, it converts the stream into a collection of discretized elements. Typically, each element is a pixel, each with an intensity that is related to its thermal levels. A microprocessor is then able to analyze the distinct elements and compare their intensity values with known temperatures from a lock-up table. This allows a digitized image to be constructed for display on a monitor. The images shown on the monitor, having been through the digitization process, are able to clearly shown an operator the presence of slag, due to its different colouration. An embodiment is also described whereby a computer would be able to analyze the contrast of the image and determine the slag content so that automated termination of the pouring can be achieved. Though this method is effective it is also expensive. As noted above, to avoid the problems caused by the omnipresent carbon dioxide and water vapours, IR wavelengths are the chosen radiation for analysis. This requires expensive and delicate imaging equipment, and also requires a delicate calibration process that, if performed incorrectly, will produce results far from the ideal. In addition, as with all electrical equipment in environments as hostile as a steel mill or foundry, there is a need for periodic recalibration, which may require the temporary cessation of activities, which is highly undesirable.
Another prior art method is illustrated in U.S. Pat. No. 4,222,506 to Sakashita et al, entitled xe2x80x9cMolten Steel Outflow Automatically Controlling Devicexe2x80x9d. This patent is specifically related to the detection of slag in the stream between the ladle and a tundish. The patent teaches the use of an infrared camera connected to a colour monitor, where the infrared image values are mapped to visible parts of the spectrum for display purposes. As the image is displayed it is analysed for the presence of yellow colouration in what should be the red stream of the steel. If more yellow appears than is defined in a threshold, an indication is given that the pouring should stop. This technique would work poorly in the BOF-to-ladle environment due to the prevalence of particulate matter, which blocks much of the information that the infrared camera is supposed to detect. In addition, gaseous matter, such as water and carbon dioxide, would interfere with the imaging to such an extent that the method would be ineffective for proper control of slag carryover. A further problem with this method is that the equipment has to be calibrated carefully so that there is a clear delineation between the molten metal and the slag. Miscalibration of the equipment will lead to performance greatly below the ideal. In addition, as a result of the environment to which the electrical equipment is subjected, periodic recalibration of the equipment is necessary, which results in additional costs.
It would be desirable to provide a method to accurately detect and display the presence of slag in the stream between two vessels, such as between the BOF and the ladle, or between the ladle and the tundish, while avoiding the use of unnecessarily expensive equipment that has a limited service life. It would further be desirable to provide such a method wherein the results of the detection process are displayed in such manner that an operator can reliably be alerted to the presence of slag so that pouring can be terminated when deemed necessary to prevent excess slag carryover.
It is an object of the invention to obviate or mitigate at least one disadvantage of previous methods and systems. It is a further object to provide a method and system for the detection of impurities in a flow of molten metal.
The present invention provides a method of representing slag in a molten metal stream comprising at least the following steps. The first step is to obtain and store a plurality of pairs of first and second digital images of the molten metal stream. The second step is to compare one or more selected properties of said stored first and second digital images of each pair which are representative of the presence or absence of slag in said stream. The third step is to obtain a molten metal flow delta on the basis of the above comparison, the delta representing the amount of slag in the stream and the rate of change in the amount of slag. The fourth step is to generate output signals which are representative of the molten metal flow delta.
The output signals may be displayed on level meters and numerical displays or used to initiate alarms indicative of the slag content exceeding a threshold or used to distinguish slag from the molten metal flow through the colourization of slag on a display.
The first and second digital images may be characterized as first and second histograms which are compared to identify changes between the histograms that representative of the presence or absence of slag. These histograms may be based on pixel intensity levels which may be adjusted on the basis of automatic gain control levels read from the digital imaging equipment.
The comparison of the digital images may be done through a process comprising as a first step, the identification of the first and second digital images respectively as first and second sets of areas of similarity, these areas of similarity being connected areas of pixels and defined on the basis of texture and intensity. The second step is to define first and second subsets of the respective first and second sets of areas of similarity, and the third step is to compare at least one selected property of each of the first and second subsets against a defined parameter outside of which parameter the property is indicative of the presence or absence of slag. Each subset is comprised of an area of similarity having an aspect ratio of height to width of said area, and the selected property comprises said aspect ratio. The defined parameter is a numerical value for the aspect ratio, in excess of which, it is indicative of the presence of slag. This threshold may be a numerical value greater than or equal to one. The histogram analysis described above may be employed after regions of similarity are defined, and the histograms are built based on at least one selected property of said first and second subsets which is indicative of the presence or absence of slag in said stream. A digital filter is applied to the first and second images prior to comparing their properties to reduce noise and digitization artifacts.
The present invention also provides a method of representing slag in a molten metal stream comprising at least the following steps. The first step is to obtain and store a digital image of the molten metal stream. The second step is to identify the digital image as a set of areas of similarity, said areas of similarity being connected areas of pixels and defined on the basis of texture and intensity of said images. The third step is to define a subset of the areas of similarity. The fourth step is to compare at least one selected property of said subset against a defined parameter for said property, outside of which parameter said property is indicative of the presence or absence of slag. The fifth step is to generate output signals based upon the results of said comparison which are representative of the presence or absence of slag in said molten metal stream.
All the above embodiments of the invention can be performed with a charge coupled device as the digital imaging device. Preferably, the digital imaging device is sensitive to wavelengths less than 4 microns, and the digital imaging device includes an optical filter, such as a neutral density filter or a cobalt blue filter.
The present invention provides a method of reducing slag carryover into a second vessel during pouring of a molten metal stream containing slag from a first vessel into the second vessel. This method comprises representing said slag in said molten metal stream in accordance with the method described above and then terminating or regulating said pouring on the basis of information upon the amount of slag in said stream derived from said output signals. The decision to terminate or regulate said pouring is made on the basis of information upon the amount of slag in said stream derived from the visual displays, or the numerical display.
The present invention also provides a system for representing the slag content of a molten metal stream comprising at least the following elements. The first element is a digital imaging device for providing a plurality of pairs of first and second digital images of said molten metal stream. The second element is a memory, operatively connected to the digital imaging device, for storing said first and second digital images. The third element is a comparator, operatively connected to the memory, for providing a comparison signal based on a comparison of one or more selected properties of said first and second digital images in each pair which are representative of the presence or absence of slag in said stream. The fourth element is a processor, operatively connected to the comparator, for receiving and processing the comparison signal to provide a molten metal flow delta which is representative of the amount of slag in said stream and the rate of change in said amount and generating output signals which are representative of said molten metal flow delta. There may be provided a visual display to which the output signals are applied, providing a numerical display of the slag content of the molten metal flow, or a visual display of said molten metal stream with regions of slag flow appearing differently from regions of metal flow upon said display, and wherein said processor selectively applies said output signals to one or the other of said regions to enhance the visual differences between said regions. These visual enhancements may be applied to the regions of slag and said regions of slag flow colourized to enhance the visual differences between said regions. The comparator may be adapted to characterize said first and second digital images as first and second histograms and to compare said histograms to identify changes between said histograms representative of the presence or absence of slag, the histograms optionally representing pixel intensity levels that may have been adjusted on the basis of automatic gain control levels read from said digital imaging device. The comparator may perform at least the following three steps. The first step is to identify the first and second digital images respectively as first and second sets of areas of similarity, said areas of similarity being connected areas of pixels, and defines said areas of similarity on the basis of texture and intensity of said images. The second step is to define first and second subsets of the respective first and second sets of areas of similarity and the third step is to compare at least one selected property of each of said first and second subsets against a defined parameter for said property, outside of which parameter said property is indicative of the presence or absence of slag. One parameter of the areas of similarity that the comparator may examine is an aspect ratio of height to width of said area. The comparator may also identify first and second areas of similarity, as above, and then builds histograms based on at least one selected property of said first and second subsets which is indicative of the presence or absence of slag in said stream.
The present invention also provides a system for representing the slag content of a molten metal stream comprising at least the following four elements. The first element is a digital imaging device for providing a digital image of said molten metal stream. The second element is a memory, operatively connected to the digital imaging device, for storing said digital image. The third element is a comparator, operatively connected to the memory, for identifying the digital images as a set of areas of similarity, said areas of similarity being connected areas of pixels and defined on the basis of texture and intensity of said images, defining a subset of the areas of similarity and comparing at least one selected property of said subset against a defined parameter for said property, outside of which parameter said property is indicative of the presence or absence of slag. The fourth element is a processor, operatively connected to the comparator, for generating output signals based upon the results of said comparison which are representative of the presence or absence of slag in said molten metal stream.
The imaging device can be a charge coupled device, or it can be sensitive to wavelengths less than 4 microns, and it may include an optical filter such as a neutral density filter or a cobalt blue filter.